Titanium dioxide is a photoactive material that is used widely as a pigment in coatings, paper plastics and ink. For pigment applications, the photoactive properties are not typically desired and the pigmentary grade titanium dioxide is generally prepared by methods that suppress the photoactivity of the material. Titanium dioxide is produced in two crystal phases, rutile and anatase, that differ in lattice structures, refractive indices, and densities. The rutile phase is the more stable phase and is favored for use in pigment applications because rutile pigments have a higher refractive index than their anatase counterparts, resulting in greater opacity and whiteness.
The anatase form of titanium dioxide is usually more photoactive than the rutile form and used for photocatalytic applications, while the rutile form is used as a pigment. The photocatalytic properties of titanium dioxide result from the promotion of electrons from the valence band to the conduction band under the influence of ultraviolet (UV) and near-UV radiation. The reactive electron-hole pairs that are created migrate to the surface of the titanium dioxide particles where the holes oxidize adsorbed water to produce reactive hydroxyl radicals and the electrons reduce adsorbed oxygen to produce superoxide radicals, both of which can degrade NOx and volatile organic compounds (VOCs) in the air. In view of these properties, photocatalytic titanium dioxide has been employed in coatings and the like to remove pollutants from the air. Such coatings may also have the advantage of being self-cleaning since soil (grease, mildew, mold, algae, etc.) is also oxidized on the surface.
International Application Publications Nos. WO2005/083014, WO 2006/030250, and WO 2005/083013 to Goodwin et al. describe self-cleaning and de-polluting coating compositions comprising photocatalytic TiO2. 
When NOx species are oxidized by the reactive species produced by the photocatalytic reaction, nitric and nitrous acids are formed. The acidic species are neutralized to nitrites and nitrates by alkaline fillers or extenders present in the coating compositions, which are removed from the coating by rainfall. The most commonly used extender is calcium carbonate.
Coating compositions that comprise photocatalytic titanium dioxide can be made using different types of organic binders or resin systems. In the absence of other materials, organic binders decompose in the presence of UV light to carbon dioxide, water and nitrogen containing species, if present, resulting in degradation of the coating. This problem is exacerbated when the coating is exposed to intense UV radiation from direct sunlight, as is the case with an exterior paint. Such coatings are often formulated with inorganic binders or with organic polymers which are resistant to photocatalytic oxidation at relatively low catalyst concentrations. Previously, coatings comprising photocatalytic titanium dioxide have been prepared with silicone-based polymers, such as siloxane polymers, due to the greater stability of these materials in the presence of active species produced from photocatalytic reactions. The use of binders exclusively comprising silicone-based polymers is disfavored because silicone-based polymers are significantly more expensive compared to other organic polymers, such as acrylic or styrene based polymers. It is desirable to prepare a cost-effective photocatalytic coating composition comprising a reduced amount of silicon-based polymer mixed with a lower cost organic polymer. However, mixing organic polymers with silicone based polymers results in lower durability of the coating composition.
Therefore, there exists a need for an improved photocatalytic coating composition that exhibits improved durability and optical properties at a lower cost, while maintaining the ability to remove acidic by-products of the photocatalytic NOx oxidation reactions.
The foregoing discussion is presented solely to provide a better understanding of the nature of the problems confronting the art and should not be construed in any way as an admission as to prior art nor should the citation of any reference herein be construed as an admission that such reference constitutes “prior art” to the instant application.